Sensors play an important part in a multitude of applications. While failures of sensors may damage machines, for example, or may lead to quality losses in products, some sensors are also used in safety-relevant applications, so that any failure or erratic behavior on their part may cause people to be insured or even to die. Therefore, there is a need for reliable sensor systems.
Sensors use, e.g., a change of electronic parameters of a device (sensor cell) due to an external influence (measured quality). For example, in a capacitive pressure sensor, the capacitance of a capacitor changes when its membrane bends due to increasing pressure. A measuring circuit accordingly measures the change in the electric parameters of a sensor cell and converts it to an output voltage or a digital value. The output voltage or the digital value is subsequently transmitted to an evaluation circuit via a signal path, and is evaluated by said evaluation circuit.
Sensor devices, such as pressure or temperature sensors having associated evaluation electronics, are frequently employed in safety-relevant applications. To verify the functionality of the sensor devices, functionality tests and, preferably, self-tests of the sensor devices are performed on a regular basis. Conventionally, self-tests of sensor devices are performed “offline”. This means that the sensor device is not operational during the time the self-test is performed. Particularly in such safety-relevant applications it is disadvantageous for the sensor device to not be operational during the self-test.
There are alternative self-test methods for, e.g., temperature sensors and pressure sensors which use an excitation of the sensors due to a temperature increase by means of a heating element or an electrostatic deflection of a capacitive pressure sensor so as to generate testable signal changes. This offers the advantage of being able to also test the sensor at the same time, but is often not acceptable due to the high level of power consumption for achieving the heat output, or due to the very high voltages for deflecting a membrane by means of electrostatics. In addition, due to the low signal energy, these methods necessitate very long observation periods until a defect is diagnosed in a reliable manner. In addition, suppressing parasitic signal paths which couple the high-energy stimulation signal into the signal path downstream from a possible defect and thus prevent the defect from being recognized, are very expensive. Parasitic signal paths in temperature sensors are, for example, the temperature dependence of the circuit of the signal path regarding a warming of the circuit IC by means of a heating element, or a crosstalk between the power supply lines, heat output drivers and heating elements, on the one hand, and nodes of the sensor signal processing circuit, on the other hand. With an electrostatic deflection of MEM capacitors (MEM=micro-electro-mechanical) with high levels of excitation voltages, parasitic signal paths may occur due to a crosstalk via a substrate or operating voltage line. A further disadvantage of these methods is that the measured value of the sensor is corrupted during the self-test. For this reason, these methods do not enable reliable operation of the sensor device during the self-test.